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Saturday, June 18, 2016

New study finds no sign of entanglement with other universes

Somewhere in the multiverse
you’re having a good day.

The German Autobahn is famous for its lack of speed limits, and yet the greatest speed limit of all comes from a German: Nothing, Albert Einstein taught us, is allowed to travel faster than light. This doesn’t prevent our ideas from racing, but sometimes it prevents us from ticketing them.

If we live in an eternally inflating multiverse that contains a vast number of universes, then the other universes recede from us faster than light. We are hence “causally disconnected” from the rest of the multiverse, separated from the other universes by the ongoing exponential expansion of space, unable to ever make a measurement that could confirm their existence. It is this causal disconnect that has lead multiverse critics to complain the idea isn’t within the realm of science.

There are however some situations in which a multiverse can give rise to observable consequences. One is that our universe might in the past have collided with another universe, which would have left a tell-tale signature in the cosmic microwave background. Unfortunately, no evidence for this has been found.

Another proposal for how to test the multiverse is to exploit the subtle non-locality that quantum mechanics gives rise to. If we live in an ensemble of universes, and these universes started out in an entangled quantum state, then we might be able to today detect relics of their past entanglement.

This idea was made concrete by Richard Holman, Laura Mersini-Houghton, and Tomo Takahashi ten years ago. In their model (hep-th/0611223, hep-th/0612142), the original entanglement present among universes in the landscape decays and effectively leaves a correction to the potential that gives rise to inflation in our universe. This corrected potential in return affects observables that we can measure today.

The particular way of Mersini-Houghton and Holman to include entanglement in the landscape isn’t by any means derived from first principles. It is a phenomenological construction that implicitly makes many assumptions about the way quantum effects are realized on the landscape. But, hey, it’s a model that makes predictions, and in theoretical high energy today that’s something to be grateful for.

They predicted back then that such an entanglement-corrected cosmology would in particular affect the physics on very large scales, giving rise to a modulation of the power spectrum that makes the cold spot a more likely appearance, a suppression of the power at large angular scale, and an alignment in the directions in which large structures move – the so-called “dark flow.” The tentative evidence of a dark flow, which was predicted in 2008 had gone by 2013. But this disagreement with the data didn’t do much to the popularity of the modelin the press.

In a recent paper, William Kinney from the University at Buffalo put to test the multiverse-entanglement with the most recent cosmological data:

Limits on Entanglement Effects in the String Landscape from Planck and BICEP/Keck Data
William H. KinneyarXiv:1606.00672 [astro-ph.CO]

The brief summary is that not only hasn’t he found any evidence for the entanglement-modification, he has ruled out the formerly proposed model for two general types of inflationary potentials. The first, a generic exponential inflation, is by itself incompatible with the data, but adding the entanglement correction doesn’t help to make it fit. The second, Starobinski inflation, is by itself a good fit to the data, but the entanglement correction spoils the fit.

Much to my puzzlement, his analysis also shows that some of the predictions of the original model (such as the modulation of the power spectrum) weren’t predictions to begin with, because Kinney in his calculation found that there are choices of parameters in which these effects don’t appear at all.

Leaving aside that this sheds a rather odd light on the original predictions, it’s not even clear exactly what has been ruled out here. What Kinney’s analysis does is to exclude a particular form of the effective potential for inflation (the one with the entanglement modification). This potential is, in the model by Holman and Mersini-Houghton, a function of the original potential (the one without the entanglement correction). Rather than ruling out the entanglement-modification, I can hence interpret this result to mean that the original potential just wasn’t the right one.

Or, in other words, how am I to know that one can’t find some other potential that will fit the data after adding the entanglement correction. The only difficulty I see in this would be to ensure that the uncorrected potential should still lead to eternal inflation.

To add meat to an unfalsifiable idea that made predictions which weren’t, one of the authors who proposed the entanglement model, Laura Mersini-Houghton, is apparently quite unhappy with Kinney’s paper and tries to use an intellectual property claim to get it removed from the arXiv (see comments for details). I will resist the temptation to comment on the matter and simply direct you to the Wikipedia entry on the Streisand Effect. Dear Internet, please do your job.

For better or worse, I have in the last years been dragged into a discussion about what is and isn’t science, which has forced me to think more about the multiverse than I and my infinitely many copies believe is good for their sanity. After this latter episode, the status is that I side with Joe Silk who captured it well: “[O]ne can always find inflationary models to explain whatever phenomenon is represented by the flavour of the month.”

34 comments:

An entanglement relationship is a correlation between measurement outcomes; since the only measurements we can perform are those that return quantities inherent in our own universe/branch, it is fundamentally impossible for us to detect either entanglement with other universes, or any unambiguous traces of it. The entire concept is meaningless, and I am with you in that all that has been ruled out here is a particular form of the inflationary potential, which allows no conclusions as to any entanglement.

For Alice in isolation, all measurement outcomes are random, and she has now way to rule out or confirm the existence of Bob, much less any correlation with his measurement outcomes, unless they classically communicate; it's no different in this case.

You don't detect the entanglement. You're basically looking for the leftover traces of a past decoherence. The idea is that this would lead to a shift of energies that affects the effective potential. Or at least that's my understanding. I have linked to the papers above, this should give you all the detail that you can possibly with for, maybe even more ;)

I can confirm that I received a notice from arXiv regarding an intellectual property complaint, and that the paper will be taken down on June 24 unless the issue is resolved:

"Dear William Kinney,

We have received a complaint from your co-workers regarding disputed authorship of arXiv:1606.00672. Our understanding is that this work was performed in a joint intellectual effort with other two researchers but they do not appear listed as co-authors of arXiv:1606.00672. Could you please explain?

Please respond to this message before the 24th June or your submission will be administratively withdrawn. "

For the record, both Dr. Mersini-Houghton and Dr. Holman were repeatedly offered authorship credit on the paper.

Appreciate offers of support, but I have responded in detail and the matter is being handled at the editorial level at arXiv, and I'm confident they will make the right decision. Please don't email them until the process plays out as it should.

Paired shoes in a spacetime left foot have different energies. They pursue divergent minimum action vacuum free fall trajectories. The smallest matter shoe is 4 atoms. A 0.113 nm³ alpha-quartz unit cell is nine atoms. Single crystal space group P3(1)21 has all atoms in right-handed helices; P3(2)21 has left-handed helices. Test spacetime geometry with a geometric Eötvös experiment[2]. Only 2×20 grams' 6.68×10^22 pairs of shoes do not cancel.

Thanks for this. As someone more attuned to the philosophic realm, I still find dabbling in these matters ('substantive' or not) stimulating on some interesting levels. I sometimes speculate on what Giordano Bruno's take on this would be, from a standpoint more in line with modern physics. Cheers

"For the record, both Dr. Mersini-Houghton and Dr. Holman were repeatedly offered authorship credit on the paper."

Sorry if this is off topic, but this situation raises an interesting general question about disputes over collaborative works. Suppose A and B write a paper together, some of the text being composed by A and some by B. Then A decides he doesn't want it published. He tells B "I don't want my name on that paper, and you can't publish my work without my consent". Obviously if it's just a matter of attribution of ideas or data, it can be easily handled with suitable acknowledgements, but Mr A might also raise copyright issues if his text is being used. Mr B may need to either replace Mr A's words with his own original words, or else invoke the "fair use" doctrine, assuming the text composed by A could be quoted in sufficiently succinct form, with suitable attribution.

Pauli and Heisenberg got into this situation once, although they didn't focus on any legalistic complaints. In 1958 they worked together on an idea for unified field theory, and planned to release a preprint. A few days prior to its scheduled release, Heisenberg couldn't resist announcing the proposed new field equation in a public lecture, and it got into the newspapers, where it was excitedly reported that "Professor Heisenberg and his assistant, W. Pauli, have discovered the basic equation of the Cosmos!" (Thankfully we no longer have such silly sensationalistic science reporting.) Shortly thereafter Pauli completely disowned the idea and refused to let his name be associated with it. He made very strong negative comments when Heisenberg presented the idea at a later conference. Heisenberg said later "Wolfgang's attitude to me was almost hostile". It's sad because they had been such close friends for so many years. They never saw each other again, and Pauli died just a few months after that conference.

Imho it is a rather perverse view about Author's rights to insist that you can use them to stop somebody else's work. These people appear to make a rather quaint claim that they are and are not authors: as authors they want the work taken down but without it they would have no rights. The multiverse is badly needed, I see.

I’m sure you are intimately aware of this, but some readers might not be. A priori, the idea of multiple causally disconnected parts of an inflating spacetime is completely different from the idea of multiple spacetimes superposed in a quantum sense. To use a discrete analogy, one could imagine a causal graph, viewed as a model of classical spacetime, which happens to possess multiple connected components, without bringing quantum-theoretic considerations into the picture at all. To use a cooking analogy, since I just burned my granola while reading your blog post, it’s like comparing one pie cut into multiple pieces with multiple pies!

On hering the article might be banned from archive I felt motivated to download it immediately.May be it's the Streisand Effect ;-)I have difficulties to understand, that archive might even consider to follow the claim. If intellectual property rights on some model, theory, formula, prediction etc. are reason to hinder publishing follow-up research and tests (of course with proper ctations) this would be the end of science. With the attempt Laura Mersini-Houghton discredits herself as scientist - or does she regard herself an artist?

" It is this causal disconnect that has lead multiverse critics to complain the idea isn’t within the realm of science."

Is this a valid criticism? We can never test what happens beyond the event horizon inside a black hole (and live to tell the tale, or publish it), but we still believe what GR tells us about this region.

Well, black holes evaporate, so their inside doesn't remain disconnected. I'd be surprised if before that was known there was no similar discussion. But the case with the multiverse is much different in that the theories that are being extrapolated are basically untested and untestable themselves. Best,

"Well, black holes evaporate, so their inside doesn't remain disconnected. I'd be surprised if before that was known there was no similar discussion."

True, but this is a classical effect. I don't know, but I guess there were similar discussions in the GR world.

Of course, the timescale for the evaporation of a large black hole is very long, so for all practical purposes (meaning that we believe now what GR says is inside) it is disconnected.

As to whether the theories which imply the multiverse (i.e. multiverses are a consequence of other theories, not hypotheses or theories themselves) are themselves untestable is another question; Tegmark would disagree with you.

To begin with what you say isn't correct, there are models of inflation which do not lead to a multiverse. Having said that though I think you didn't understand my reply. I said I consider it a good explanatory model for observations, not more and not less. I mean by that, it's a mathematical framework to calculate what we observe, and to the extent that it contains other universes these are also mathematical tools to make calculations. Best,

Dr Bee - I did understand the nuance of what you were saying. I think it's a good holding position. Excellent actually.

I also think it's worthwhile putting that stance under a littler pressure, if the vehicle for doing so, offers a better resolution of one or more aspect in background knowledge.

"To begin with what you say isn't correct, there are models of inflation which do not lead to a multiverse"

- There aren't. - The solution framework that has been in play since the 1980's, predates realizations about Inflation Theory, that it is generically multiversal.

"I thought someone would figure out a resolution once the problem was revealed. That was 1983. I was wrong. Unfortunately, what has happened since is that all attempts to resolve the multiverse problem have failed and, in the process, it has become clear that the problem is much stickier than originally imagined. In fact, at this point, some proponents of inflation have suggested that there can be no solution. We should cease bothering to look for one. "

Paul Steinhardt - co-founder Inflationary Theory.

He only mentions 'some proponents' which is much too weak to infer anything (and nothing is). The purpose of the quote is actually this segment "...We should cease bothering to look for one".

The quote is taken from a Paul Hogan, Paul Steinhardt interview. It was actually yu that turned me onto Hogan, so it's more than likely back-reaction sent the original article my way.

Guth has recently all but admitted it. In a statement he said he still thought there was a non-multiverse Inflation Model out there, and they weren't going to abandon looking or it. But they can't find it. Alan Guth recently said the search was continuing, and that he thought one or more non-multiversal solution was probably 'out there'. They just can't find it.

My understanding was that it's possible to finetune the potential parameters so that you don't get eternal inflation. It's possible that this turned out to be wrong, but honestly it would surprise me if someone showed (or even argued) that any possible potential for any type of field(s) must lead to eternal inflation, so I'd be very interested in a reference.

Then there's hybrid inflation. (Not saying that I'm a big fan of any of that.) I have to admit I'm kind of dramatically uninterested in this whole discussion. You brought this up in relation to the quote by Silk - could you maybe just let me know what point you're trying to make and save us both time? Best,

Sabine, my apologizes for putting this question to this unrelated thread, it is much more down to earth than multiple universes but still entirely hypothetical:

My understanding is that any accelerated masses that exhibit a quadrupole moment will emit gravitational waves/radiation. Does this mean that it is save to assume that electrons of atoms/molecules with an electric quadrupole moment must also emit minute gravitational radiation (obviously beyond anything we could ever hope to measure)?

Erm, sorry, now you've totally lost me, I have no idea what you are even commenting on. As I thought I said very clearly above, for me inflation is a model that helps fit the data. So far so good. I explained here that it's possible the multiverse idea can be used to arrive at deeper insights. I just consider this extremely unlikely, for the reason (as I expressed through the quote by Silk) that at least in the present versions it's too ambiguous to actually predict anything. Which is exemplified by the paper I discussed in this blogpost. Hope that clarifies it.

Please refrain from posting such off-topic comments. I'll make an exception for this one, but I'll not approve further comments. The brief answer to your question is "yes" though there are of course, as always, certain assumption that enter such an extrapolation. You shouldn't think of the system as continuously emitting gravitational radiation but as of having an extremely small probability to suddenly emit a graviton. And, yes, there are people who have estimated this cross-section etc - as you say it's ridiculously tiny and unobservable. Best,

again sorry for having gone rouge, and thanks for making an exception. I've been searching for a paper that would do these calculations, but didn't think of throwing the graviton and cross-section in there to get a good match. With your answer a search quickly takes me to a relevant paper.

A quick update: arXiv is leaving the paper up (for now). For those who have kindly offered to help with letters of support and the like: Sabine is right about the Streisand Effect. Regardless of how it shakes out, I hope that the fact that this happened at all becomes widely known and enters the public discussion. Openness makes science work, so shine some daylight! (Wouldn't hurt if you cite the paper, either.)